1. Field of the Invention
The embodiments discussed herein are related to a power semiconductor module using a power semiconductor element that controls high current and high voltage.
2. Background of the Related Art
A power semiconductor module includes a plurality of power semiconductor elements, and is used as, for example, a power conversion element of an inverter apparatus. As the power semiconductor element, there are a metal oxide semiconductor field effect transistor (MOSFET), an insulated gate bipolar transistor (IGBT), a freewheeling diode (FWD), and the like. Further, as the power semiconductor element, there are a reverse conducting (RC)-IGBT formed by integrating the IGBT and the FWD, and a reverse blocking (RB)-IGBT having a sufficient withstand voltage against reverse bias.
In such a power semiconductor module, the semiconductor element has an electrode on a rear surface thereof which is bonded to an insulating substrate by solder and a front-surface-side electrode, through which main current flows, bonded to a wiring conductor by solder. Here, it is well known to use solder with different physical properties for the rear surface and the front surface of the semiconductor element (e.g., refer to Japanese Laid-open Patent Publication No. 2006-287064 and Japanese Patent No. 4730181).
It is an object of the semiconductor device in Japanese Laid-open Patent Publication No. 2006-287064 to realize a bonding portion which is excellent in heat resistance and thermal fatigue resistance. To this end, as solder paste for bonding the semiconductor element and the conductor of the insulating substrate, “Sn(tin)-3.5Ag(silver) -0.5Cu(copper), (melting temperature: 220° C.” is used. Further, as cream solder for bonding the wiring conductor and the semiconductor element, “Sn-20Ag-5Cu (solidus line temperature: 220° C. and liquidus line temperature: 345° C.)” is used. In addition, also taking into account the heat release to the front surface side of the semiconductor element, the wiring conductor bonded to the semiconductor element is made of a copper material whose bonding portion to the semiconductor element is thick and also large in area, and has a wiring portion extending out from the top end thereof in the surface direction of the semiconductor element.
Japanese Patent No. 4730181 discloses a semiconductor device in which a semiconductor element is sandwiched by two conductor members and the freezing point of the solder that reaches the lower side of the semiconductor element when the solder reflows is set to be lower than the freezing point of the solder that reaches the upper side of the semiconductor element.
Chips having mounted thereon semiconductor elements used for power semiconductor modules are made thin, and silicon substrates with a thickness of 100 micrometers (μm) or less have become available. Further, the semiconductor elements are each bonded by solder to an insulating substrate as a supporter thereof and to a lead terminal as a wiring conductor. Furthermore, the thus configured semiconductor element, insulating substrate, and lead terminal are sealed with epoxy resin, for example.
The lead terminal is thermally expanded by heating of the semiconductor element. At this time, the lead terminal is pressed by sealing with resin and is therefore expanded toward the thin semiconductor element. As a consequence, high stress is applied to the semiconductor element from the lead terminal, and this causes generation of cracks on a surface electrode of the semiconductor element and may cause breakage thereof.